1. Field of the Invention
Embodiments disclosed herein generally relate to compositions and methods for removing a filter cake from a wellbore. More specifically, embodiments disclosed herein relate to the use of an energized fluid breaking a filter cake or for generating a self-cleaning filter cake.
2. Background
When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining wellbore stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, fluid used to place a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
During drilling, the pressure balance between the circulating drilling fluids and that of the formation being drilled may be maintained in an underbalanced or an overbalanced mode. Underbalanced drilling is a method of drilling a desired subterranean formation where the hydrostatic pressure exerted by a column of drilling fluid in the drill string is less than the natural pressure (pore pressure) inherent in the subterranean formation being drilled. Underbalanced drilling may prevent damage to the desired subterranean formation and in particular low pressure formations. Typically, the pressure differential is set to provide a margin above the pressure at which wellbore collapse might occur. The introduction of sufficient air, nitrogen or other gases to the drilling fluids can reduce the density of the commingled fluids and effectively decrease hydrostatic pressure. Other low density fluids such as emulsions, foams and mists may be used as a drilling fluid to achieve an underbalanced condition.
In overbalanced drilling, fluid in an annulus of a well is used to exert a pressure that is greater than the formation pressure. The mud weight, or density, may be calculated to give the appropriate pressure gradient across the exposed formation to provide the optimum fluid migration rate into the least stable horizon of the exposed formation. Thus, the pressure that is exerted by the annular fluid keeps formation fluids from exiting the well and may provide support for the wellbore. A drawback to this technique is that particulates added to increase the weight of the fluid (and, thus, increase its downhole pressure), as well as other particulates, emulsified fluids and surfactants, may be pushed in and damage the formation. The well may also need to be tested after overbalanced drilling to check for formation damage.
In addition to the appropriate use of underbalanced or overbalanced drilling, another way to protect the formation is by forming a filter cake on the surface of the wellbore. A filter cake is a tough, dense, practically insoluble residue that reduces the permeability of the formation and which is formed when particles or emulsified fluids suspended in a drilling fluid coat or plug the pores in the subterranean formation while drilling overbalanced. Filter cakes may be formed a number of ways known in the art, including the use of both clay and non-clay based drilling fluids.
Sealing off producing formations using a filter cake may also be desired in order to prevent fluid loss and possible damage to the formation. Filter cakes can prevent loss of drilling fluids to the formation by substantially preventing fluids from passing between the wellbore and the formation. Formation of a filter cake may also be desired prior to completion or workover of a well.
It is often desirable or necessary to remove the filter cake prior to cementing or bringing a well on production. The presence of the filter cake can hinder the passage of fluid from the formation to the wellbore and thereby retard production rates. Where a fluid or gas is being pumped into the formation for enhancing oil recovery, a filter cake can hinder the passage of fluid from the wellbore to the formation, thus hindering recovery efforts. Therefore, removal of filter cake is necessary to increase the flow of fluids from or to the formation, as required. Since filter cake is dense and practically insoluble in aqueous fluids, and generally adheres strongly to the formation, it cannot be merely flushed out of the formation. Removal of filter cake requires some additional treatment.
Various methods have been used to clean up filter cakes, including techniques invoked during flowback of producing fluids. For example, see Zain, Z. et al, SPE Drilling & Completion, December 2001, and special materials in fluid loss pills, e.g. SEAL-N-PEEL®.
One method to break or clean up the filter cake involves spotting an oxidizing agent or enzyme to destroy organic constituents in the cake and an acid to react with carbonate bridging agent (Luyster, M. R., SPE 58749). U.S. Pat. No. 6,861,394 discloses a wellbore fluid including a peroxide-degradable polymer and an encapsulated peroxide source. The release of peroxide from the peroxide source is controlled by means of pH such that the peroxide source can be activated, and peroxide released, by a change in pH. In a wellbore, this pH change can be effected by using produced fluids to lower the pH of a more alkaline wellbore fluid. The peroxide, when released, degrades the polymer and lessens the integrity of the filter cake.
U.S. Pat. No. 5,251,697 discloses the addition of calcium carbonate to water being injected into a well. The calcium carbonate particles either clog the pores in the subterranean rock formations or collect and build a filter cake. When the filter cake is to be removed, the '697 patent directs the operator to circulate an acid wash, preferably hydrochloric acid, into the well. The acid wash will dissolve the calcium carbonate and thereby destroy the filter cake. At this point the well can be brought on production or additional work may be performed on the well.
U.S. Pat. No. 5,238,065 discloses the use of a peroxide-degradable polymer and an encapsulated peroxide source in the drilling fluid, which forms a filter cake containing both of these elements. The peroxide degradable polymer may be a polysaccharide, and the peroxide source may be an inorganic peroxide, including zinc and alkaline earth metal peroxides, such as magnesium peroxide. The encapsulating material for the peroxide may be a polymer, including enteric polymers. The release of peroxide, from peroxide sources generally, can be controlled by reduction of pH. In this case, when it is time to remove the filter cake, the pH of the fluid simply needs to be reduced or the low-pH formation fluid needs to be brought in contact with the filter cake to activate the peroxide, The latter degrades the peroxide-degradable polymer and causes the filter cake to lose its integrity and fall apart. The encapsulated peroxide is a member of the class of materials generally referred to as internal breakers. The use of an internal breaker is beneficial because it requires less peroxide, less loss of wash fluid to the formation, and gives more complete removal of the filter cake.
U.S. Pat. No. 6,886,635 discloses a composition for a filter cake removal fluid having a persulfate salt. The persulfate salt breaks down the filter cake in a controlled manner at downhole temperatures ranging from about 65° F. to about 165° F. without the addition of activators. Mud additives that generate acid in situ upon being exposed to a triggering event, such as a critical temperature or salt concentration, may also be used (Nasr-el-din, H. R. et al, SPE 96965).
Noncorrosive chelating agents, such as those employed in the Schlumberger MUDSOLV® process, can be even more effective than acid removal of the bridging agent. Some of the chelating agents can also be encapsulated and added to the mud so that they become incorporated in the filter cake. Before the well comes on production, the reagents are released by introducing a triggering agent at the face of the cake, such as a change in pH. Other systems treat a filter cake containing carbonate particles with glycol, such as in the SEAL-N-PEEL® system available from MI-SWACO®, where the glycol reduces the adhesive forces of the carbonate, allowing the filter cake to break.
Enzyme systems are known to degrade the types of polysaccharides used in fracturing, blocking gels and other oil industry applications. Enzyme breaker systems have been designed to break gelled fracturing and blocking fluids used in the industry. See, for example, U.S. Pat. Nos. 5,201,370 and 5,224,544. Enzymes, for example the cellulases, hemi-cellulases, amylases, pectinases, and their mixtures, are familiar to those in the well service industry when used in fracturing gel breaker systems. Some of these enzymes break the bonds that connect the monosaccharides into a polysaccharide backbone, for instance, the (1,4)-α-D-galactosiduronic linkages in pectin. These conventional enzymes are nonspecific mixtures that cause random breaks. Therefore, using these conventional enzymes to break gelled fracturing fluids results in only a partial degradation of the polysaccharide polymer. Instead of fragmenting almost completely into much smaller fragments, these enzymes break the polysaccharide backbone into a mixture of fragments consisting of monosaccharides, disaccharides and polysaccharides. Larger crosslinked fragments like disaccharides and polysaccharides can remain behind and damage the production zone. Since the breaks are nonspecific, conventional enzymes also can degrade other components used in the system.
U.S. Pat. No. 5,247,995 discloses a method of degrading damaging material within a subterranean formation of a well bore using an enzyme treatment. Filter cakes and very viscous fluids are such damaging materials. The enzyme treatment degrades polysaccharide-containing filter cakes and damaging fluids, thereby reducing their viscosity. The degraded filter cake and damaging fluid can then be removed from the formation back to the well surface with less back pressure. The particular enzymes utilized are specific to a particular type of polysaccharide and are active at low to moderate temperatures. The enzymes attack only specific linkages in filter cakes and damaging fluids and are active in the pH range of about 2.0 to 10.0.
Drilling fluids may also employ oil-based surfactants or hydrophobic insoluble materials in the mud that become incorporated in the filter cake, such as the FLOTHRU™ drill-in fluid available from MI-SWACO®. During production, hydrocarbons interact with the surfactants or hydrophobic filter cake components to create minute channels through the filter cake. Invertible emulsions, which may aid in filter cake removal, are also described in U.S. Pat. Nos. 6,828,279, 6,822,039, and 6,806,233, for example.
The above described systems have drawbacks, including premature activation of the degradation mechanism. Premature activation can result in premature weakening of the filter cake, as well as reduction in the ability of the filter cake to control fluid loss. Additionally, the acid or other solutions used to dissolve filter cakes can have a harmful effect on the formation, if they are not prevented from invading it.
As another drawback, common oxidants, for example persulfates, are often ineffective at low temperatures ranging from ambient temperature to 130° F. In this temperature range, the oxidants are typically stable and do not readily undergo homolytic cleavage to initiate the degradation of the filter cake. Cleavage is typically achieved at lower temperatures only by using high concentrations of oxidizers. However, oxidizers frequently have very limited solubility, and usually it is not possible to maintain high concentrations of oxidizers in solution.
Reactions with common oxidants are also difficult to control. Common oxidants break polysaccharides into nonspecific units, creating a filter cake consisting of a mixture of monosaccharide, disaccharide, and polysaccharide fragments as well as other miscellaneous fragments. Oxidants can also react with iron found in the formation, producing iron oxides which precipitate and damage the formation, thereby decreasing permeability. Oxidants can also react nonspecifically with other materials used in the oil industry, for example, tubing, linings, and resin-coated proppants.
Further, oxidants can break down any subsequent gels used in the formation. If the oxidants are not completely removed or inactivated, they can prematurely break the new gel. Therefore, oxidants must be completely removed or inactivated before subsequent introduction of another gel into the subterranean formation.
Accordingly, there exists a need for improved processes for removing a filter cake from a wellbore.